As fastener joints in work pieces have become more highly stressed, especially in the aerospace industry, a great deal of effort has been expended in an attempt to increase the fatigue life of the fastener joints. One of the most successful techniques that has been used is commonly referred to as coldworking in which the holes through the work pieces are expanded beyond the elastic limit of the material of the work pieces prior to the installation of the fastener through the holes. This causes a compressive stress gradient to be induced in the work pieces immediately adjacent the holes that serves to increase the fatigue life of the resulting joint. Several techniques have been conceived which coldwork the holes such as those disclosed by U.S. Pat. Nos. 3,270,410; 3,434,327; 3,566,662; 3,805,578; and 3,835,688. All of these techniques use a single piece mandrel with a continuous machined and ground expansion surface thereon that is usually polished and which is pulled or pushed through the holes either with or without a sleeve member.
Because these mandrels have been made of one piece, however, it has been difficult to achieve the necessary hardness in the enlarged portion of the mandrel which contacts and expands the hole while at the same time being able to achieve the necessary toughness in that portion of the mandrel which is used to pull the mandrel through the hole. As a result, the prior art has had to resort to the use of exotic lubricants and to the use of sleeve members to isolate the surface of the hole from the expansion surface of the mandrel in order to be able to physically pull the mandrel through the hole and also not damage the hole. Also the expansion surface has frequently become damaged requiring it to have to be resurfaced by machining, grinding, and/or polishing operations or the mandrel thrown away.
Because of the critical dimensional tolerances that must be provided in the mandrel, the continuous expansion surfaces were usually made by first rough machining the surface followed by a grinding operation to preliminarily shape the surface. The grinding operation was usually followed by a honing operation which finally sized and shaped the surface and then the surface was finally polished to prepare it for the coldworking operations. All of these manufacturing steps make the cost of preparing the expansion surface on the mandrel relatively expensive. This has resulted in a reluctance on the part of the industry to use the technique of coldworking the hole unless it was determined to be absolutely necessary in order to limit the production cost of the item which is to be coldworked.